bump on this point.
On 6.0 beta 1, the pictures of album info are fetched by unsecure HTTP even if subsonic is accessed via HTTPS.
e.g.
http://img2-ak.lst.fm/i/u/3f47091dff854 ... 99b261.pngFirefox gives warnings like
Loading mixed (insecure) display content "http://img2-ak.lst.fm/i/u/3f47091dff85400ebd40beb14b99b261.png" on a secure page[Learn More] jquery-1.7.1.min.js:2:28617
GET
http://img2-ak.lst.fm/i/u/3f47091dff854 ... 99b261.png [Mixed Content] [HTTP/1.1 200 OK 567ms]
and load the image anyway.
Mixed content seems bad (I'm not a security specialist though), so a solution would be to fetch all content by HTTPS but unfortunately, for img2-ak.lst.fm the certificate is for akamai, not lst.fm, giving a SSL error. Maybe there is a direct link with working SSL? Would be the easiest fix.
Or, as OP suggested, to fetch the image on the server-side, then save it and deliver it to the client. A bit heavy maybe.